The present invention relates to blast furnace sinter of higher than normal MgO content, and particularly to production of high MgO sinter having substantially the same strength as low MgO sinter.
Iron-bearing particulates which would otherwise be too fine for use as a charge material for the iron blast furnace can be made suitable for such use by the process of sintering. Sintering involves heating a mixture of the iron-bearing particulates and various other materials to an elevated temperature at the point of incipient fusion of the iron, causing agglomeration of the material into coarse lump form. The primary constituent of the mixture is usually iron ore although other iron-bearing materials can be used. The latter include ironmaking and steelmaking revert materials such as flue dust, mill scale, sinter returns, and various other recovered wastes containing iron. Fuel is usually added to the sinter mix in the form of coke breeze or other carbonaceous material. Finally, the mixture usually includes a slag former constituent to provide, at minimum, certain slag-forming compounds which are required to obtain strong bonding of the particulates and enhance the strength of the sinter produced. In this respect, sintering can be distinguished from the related process of pelletizing to which the present invention is not applicable by the presence of higher amounts of slag-forming compounds in the sinter mix and the more rapid rate of heating and shorter time at elevated temperature than are used in the mix and treatment for induration of pellets. The sinter mix generally must include at least 16% by weight, and usually more than 20%, of the sum of CaO+MgO+SiO.sub.2 +Al.sub.2 O.sub.3, wherein for pelletizing the total level of these compounds is less than 10 percent.
It is also known to add some of the basic slag-forming materials needed in the blast furnace to the sinter mix so as to provide this material in the sinter itself. Limestone is commonly added in various amounts for this purpose. Dolomite also may be added to provide some of the MgO required in the blast furnace slag. However, it is well-recognized that dolomite additions tend to decrease both the production rate of the sintering process and the strength of the sinter produced. Generally, the decrease in sinter strength has been attributed to the formation of magnesium ferrite, a bonding agent comprised of FeO and MgO, which is more refractory than calcium ferrite. Attempts to compensate for the higher melting point of magnesium ferrite by adding increased fuel to the sinter mix have not proved successful in that the strength of sinter produced was not increased. It was also thought by some that the lower strength was due to dolomite being less reactive than limestone because of its larger crystal size (on a microscopic level) than that of limestone. On this basis, it was proposed that certain dolomites which have a relatively smaller crystal size than others should provide higher strength. However, the use of smaller crystal size dolomite has not provided sinter with strength levels approaching that of limestone sinters.
It is therefore a primary object of this invention to provide a process for producing a sinter having higher than normal MgO content with substantially the same strength level as low MgO sinter.